The present invention relates to a lock-up control system for a torque converter provided in a power train such as an automatic transmission including a continuously variable transmission. Specifically, the present invention relates to a torque converter lock-up control system that shifts the torque converter from a converter state in which a relative rotation speed between input and output elements of the torque converter, that is, an actual slip rotation speed of the torque converter is not restricted, or from a slip control state in which the actual slip rotation speed is limited to that conforming with a target slip rotation speed, to a lock-up state in which the input and output elements are directly coupled to each other and the actual slip rotation speed is zero.
A torque converter conducts power transmission between the input and output elements via fluid to thereby perform a torque fluctuation absorption function and a torque multiplication function but have a reduced torque transmission efficiency. For the reasons discussed above, recently, automotive vehicles often use a lock-up torque converter in which the input and output elements (pump and turbine elements) can be directly coupled with each other by a lock-up clutch under engine/vehicle operating conditions that do not require the torque fluctuation absorption function and the torque multiplication function, or a slip rotation speed of the lock-up torque converter, that is, a speed difference between the input and output elements can be restricted by slip control of the lock-up clutch depending on engine/vehicle operating conditions.
Japanese Patent Provisional Publication No. 2004-324847 discloses a lock-up control device as a conventionally known lock-up control technology. In the conventional lock-up control device, an actual slip rotation speed of a torque converter is gradually decreased by advancing engagement of a lock-up clutch that is disposed between input and output elements of the torque converter. After the actual slip rotation speed is decreased to a predetermined value, a lock-up differential pressure command value (a lock-up control command value) for controlling engagement operation of the lock-up clutch is abruptly and stepwisely changed so as to promote shifting of the torque converter to the lock-up state and reduce a lock-up time required to achieve the lock-up state.
Referring to FIG. 3, the conventional lock-up control is explained. As shown in FIG. 3, at moment t1, accelerator opening (accelerator position) APO of an accelerator is increased by depressing the accelerator pedal as indicated by solid line to thereby increase engine torque Te as indicated by broken line. At this moment t1, an operating region of the torque converter is shifted from a converter region in which an actual slip rotation speed of the torque converter should not be restricted, to a lock-up region in which the actual slip rotation speed of the torque converter should be decreased to zero.
Upon the transition from the converter region to the lock-up region, at moment t1, the lock-up differential pressure command value for the lock-up clutch is stepwisely increased in order to reduce backlash of the lock-up clutch. After moment t1, the lock-up differential pressure command value is gradually increased at a predetermined rate of change with time by feedback control or feedforward control such that actual slip rotation speed |Ne−Nt| of the torque converter is gradually decreased at a target gradient with respect to time which is set so as to suppress occurrence of a lock-up shock. Actual slip rotation speed |Ne−Nt| of the torque converter is given as a speed difference between engine speed Ne, i.e., torque converter input rotation speed Ne, and turbine rotation speed Nt, i.e., torque converter output rotation speed Nt.
By conducting the lock-up control on the basis of the lock-up differential pressure command value, engagement of the lock-up clutch proceeds at a rate corresponding to the change in the lock-up differential pressure command value, so that actual slip rotation speed |Ne−Nt| of the torque converter is decreased as shown in FIG. 3. At moment t2, actual slip rotation speed |Ne−Nt| of the torque converter reaches predetermined slip rotation speed value ΔNs that is set in order to judge whether actual slip rotation speed |Ne−Nt| of the torque converter becomes equal to a slip rotation speed at which a considerably large engagement shock does not occur even when the engagement of the lock-up clutch abruptly proceeds. At moment t3 at which predetermined time period Δt set for stabilizing the lock-up control has elapsed from moment t2, the lock-up differential pressure command value is rapidly and stepwisely increased to a maximum value as an upper limit as indicated by two-dot chain line “a” in FIG. 3.
By thus stepwisely and rapidly increasing the lock-up differential pressure command value, the engagement of the lock-up clutch can abruptly proceed so that the actual slip rotation speed can be rapidly decreased toward zero to thereby correspondingly reduce the lock-up time. Further, even when the engagement of the lock-up clutch abruptly proceeds, actual slip rotation speed |Ne−Nt| is not more than predetermined slip rotation speed value ΔNs, whereby the engagement shock does not become considerably large.